I've used one (https://www.carradice.co.uk/bags/saddle-packs-sqr-bags/super-c-sqr-slim) for credit card touring for quite a few trips. I think it's pretty much perfect. No fussing about with straps, it is easy to carry off the bike and it doesn't swing on the bike. The only thing that you have to do is put some gorilla tape around your seat post to make it clamp nice and tight and not damage your seat post. I didn't have a bar bag for those trips but I have a rando bag now and I would take that for future trips. I like the look of the more traditional Carradice bags but I've used this to carry laptops, locks, shoes etc and I can't really fault it.
I used the Restrap saddle bag for two commutes to test it out before a credit card tour. It was not great. The outside thing developed a hole as it rubbed on the seat tube collar, the dry back ripped and it swayed about. I sold it and bought an Apidura for the trip instead.
I used the Apidura cross country seat pack to cycle to France on a five-day credit card tour. I had a bar bag too. It packed nice and neatly, was robust, not too much sway but was a pain to get on and off. I've still got it as it would be better off road than the Carradice but I've not used it since.
Edit: The 16 litre capacity is spot on. It carries loads of stuff.